drm/amd/display: prefer preferred link cap over verified link settings

[why]
when preferred link cap is set, we should always use
preferred in all validation.
we should not use preferred for some validation but use
verified for others.

[how]
create getter function that gets verified link cap.
if preferred is set, return preferred link settings instead.

Signed-off-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Nikola Cornij <Nikola.Cornij@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>